I. Field of the Invention
The present invention concerns coating compositions and methods of use therefor. More particularly, the present invention concerns coating compositions for rendering metallic parts corrosion resistant and methods of use therefor. Even more particularly, the present invention concerns coating compositions useful in rendering metallic parts resistant to salt spray corrosion and methods of use therefor.
II. Prior Art
The oxidation or rusting of metallic parts upon prolonged exposure to the elements is well documented. As an industrial problem, oxidation problems are many-fold. A particularly significant industrial problem occurs in the automotive industry when oxidation of metallic parts of a vehicle, such as bolts, nuts, etc. occur. First, there is the replacement problem. Oxidation or rusting, when coupled with galvanic welding, may make it virtually impossible to remove and replace a rusted bolt or nut, thereby necessitating the need to burn it off with a torch or similar tool. Likewise, a bolt or nut may become so oxidized as to break. This, in turn, can rupture or weaken a significant structural component thereby leading to potential injury and/or accident with the vehicle.
To alleviate the problem of rusting fasteners and other components of a vehicle, the automotive manufacturers have established certain minimum corrosion standards. Unless a fastener can meet these standards, it is unsuitable for use. The most common industry test is a salt spray corrosion test wherein a fastener is exposed to a salt spray atmosphere, for a pre-determined time. The extent of corrosion of the part after the predetermined time period determines if the part is acceptable or not.
Therefore, fasteners expected to meet automotive industry corrosion resistance standards have conventionally been treated with a phosphate solution, such as zinc phosphate or the like, in accordance with well-known techniques. Thereafter, the part is coated with a "phosphating oil" or other salt-resistant coating. Yet, such so-treated parts have great difficulty in passing corrosion-resistance tests. The present invention, thus, seeks to overcome this problem in the prior art by providing a coating composition for impairing corrosion resistance to metal fasteners and other parts which can successfully pass an automotive industry test. As will be evident from the ensuing disclosure, the composition hereof is distinct from prior art compositions. For example, U.S. Pat. No. 4,216,032 teaches an improved "phosphating oil" wherein an effective amount of polyvinylpyrrolidone and zinc powder is added to the oil.